Section 2.5 Geology as Science
Geology is a systematic study Earth materials and processes that shape the planet. This includes exploring:
the structure and composition of Earth materials,
the physical processes and forces shaping the Earth's surface, and
the formation and history of the Earth system
Broadly speaking, geology is often divided into major categories: physical geology, which examines Earth materials and seeks to understand the many processes that operate on our planet, and historical geology, which seeks an understanding of the origin of the Earth and its development though time. The science of geology (and scientific inquiry in general) assumes that the world is comprehensible (that is, humans can perceive and make some sense of the world around us) and consistent (that is, although given freedom, it operates with regularity according to natural laws).
Principles of Geology
Since the rise of science, there have been two basic philosophic principles describing how geologic forces have shaped the Earth:
catastrophism is the belief that the landscape has been shaped primarily by great catastrophes. This was the prevailing view until the early 1800s.
uniformitarianism is the belief that the landscape has been shaped by everyday processes acting over vast periods of time (this view is often summarized as "the present is key to the past"). This approach was first proposed by Hutton (1795) and popularized by Lyell (1830).
It should be noted, however, that these represent end-member perspectives and that the modern Earth and planetary sciences recognize the role of both catastrophic and uniform events operating in geologic systems.
We can also introduce a set of geologic principles that are used to interpret a given landscape:
principle of original horizontality: sedimentary layers are originally deposited in horizontal layers.
principle of lateral continuity: sedimentary layers originally extend laterally in all directions; rocks that are separated may have been originally comprised the same layer.
principle of superposition: in stratigraphic sequence of rock layers, the oldest rock layer is at the bottom of the sequence and the youngest is at the top of the sequence.
principle of cross-cutting: the geologic feature that cuts or crosses another is the younger of the two features.